Adjustable magnetically focused triode

ABSTRACT

The output power of a magnetically focused thermionic electron tube is varied by altering the strength of the magnetic focusing field. In one embodiment, the output power of an oscillator using a magnetically focused triode may be reduced over a wide range by reducing the focusing field without any detrimental increase of intercepted beam current. The field is reduced by use of a magnetic shunt across the pole pieces of a permanent magnet used for focusing, the shunt being moved mechanically toward or away from the pole pieces. The device may also be used to control gate current in a gate controlled tube.

[111 3,826,945 [451 July 30, 1974 ADJUSTABLE MAGNETICALLY FOCUSED Primary Examiner-Herman Karl Saalbach Assistant ExaminerSiegfried H. Grimm TRIODE Attorney, Agent, or Firm-John T. OHalloran; Menotti .l. Lombardi, .lr.; Edward Goldberg arm BOO

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we SN er m0 IC s A no 7 ABSTRACT [22] Filed: Feb. 28, 1973 [2|] Appl. No.: 336,656

The output power of a magnetically focused thermionic electron tube is varied by altering the strength of the magnetic focusing field. In one embodiment, the output power of an oscillator using a magnetically focused triode may be reduced over a wide range by reducing the focusing field without any detrimental increase of intercepted beam current. The field is reduced by use of a magnetic shunt across the pole pieces of a permanent magnet used for focusing, the shunt being moved mechanically toward or away from the pole pieces. The device may also be used to control gate current in a gate controlled tube.

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PATENTEuJuLamsm 3,826,945,

sum 1 or 4 PRIOR ART PRIOR ART PATENTEDJULBOISM SHEU 2 [IF 4 PATENTEDJULSOIBM SHEEI '4 OF 4 ADJUSTABLE MAGNETICALLY FOCLSEI) TRIODE BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to magnetically focused thermionic power tubes and particularly to a novel structure for adjusting the output power thereof.

2. Description of the Prior Art In such electron tubes. while the anode current is controlled by means of the potential applied to a control grid or gate electrode. the beam is magnetically focused from a cathode to an anode so as to avoid. so far as possible. interception by control electrodes. A basic requirement of many industrial heating oscillators. for -which a magnetically focused thermionic tube is eminen tly suitable. is to provide a continuous control of power output over a wide range. Thyristor controls may be incorporated for adjustment of the potentials applied to the tube electrodes. but this is expensive and does not have a smooth output such as is required for some applications. Other methods such as the use of rectifier controls and variable transformer output coupling are not only expensive but are usually lacking in fine control and range of regulation. A known type of magnetically focused triode is described in British Pat. No. I.l95.7(l3 of the same inventor as the instant application.

SUMMARY OF THE INVENTION According to one aspect of the present invention a thermionic gate-controlled power tube has a pennanent magnet structure arranged to produce a magnetic focusing field which may be used to minimize gate current. The magnet strucure is so constructed that during use of the tube. such as in an oscillator circuit. he strength of the field can be adjusted to vary the power output of the tube over a useful range without causing cessation of oscillation.

Although it might be expected that reduction of the strength of the magnetic focusing field would lead to serious defocusing of the electron beam and impermis sible increase in grid current. this has been found not to be the case but. rather. a very large variation in output power may be obtained solely by adjustment of the strength of the magnetic focusing field without proportionately large variations in current interception of the beam.

In preferred embodiment of the invention the mag netic field is produced by a magnet with the tube mounted between facing magnet 'polepieces. The strength of the magnetic focusing field is then mechanically controlled by adjustment of the position of a magnetic shunt with respect to the polepicces to shunt an adjustable proportion of the magnetic flux flowing across the main air gap between them. in which air gap the tube is situated.

According to another aspect of the present invention there is provided for a thermionic gate-controlled power tube. a magnet structure comprising a substantially U-shaped magnet having facing polepieces on opposite sides of the position occupied by the tube. A shunting part of magnetic material and a control mem ber connected with the shunting part are operable to move the shunting part toward or away front the polepieces to shunt the magnet to a lesser or greater extent as desired so as to vary the strength of the magnetic field whereby. with the tube in the air gap and in use. such as in an oscillator circuit. the power output of the tube is adjustable.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will be described with reference to the accompanying drawings in which:

FIG. I is a diagrammatic representation of the electrodc configuration of a magnetically focused triode FIG. 5 is an oscillator circuit diagram incorporating the tube of FIGS. 1 and 2 and the structure of FIG. 4. and

FIG. 6 is a graph showing the variation in output power with magnetic focusing field obtained with the circuit of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT A diagrammatic representation of the electrode por tion of a known magnetically focused tube. such as in the above British Patent. is shown in FIG. 1. Filamentary thermionic cathodes 10 are mounted parallel to one another midway between and parallel to the opposite broad walls of a rectangular anode IL A set of grid plates I2 is mounted with the plates edgewise on to the broad sides of anode 11 so that each cathode 10 is situated symmetrically between a pair of the grid plates. The grid plates are each mounted on a respective rod 13 secured in a metal support plate 14. In use. a magnetic field H.'denoted by the arrow. is directed normal to the broad sides of the anode and parallel to the grid plates, 12 so that electrons accelerated towards the. anode between the grid plates are focused on the anode without any substantial interception by the grid plates. even when they are driven positive. Fundamentally the tube operates as a grid-controlled tube with the grid plates providing equipotential surfaces on either side of each cathode filament. If desired the grid plates may be replaced by respective aligned sets of rods parallel to one another and to the cathode filaments. Additional rods between the edge of each grid plate and the facing anode wall may be incorporated to provide a second grid as in a beam tetrode. The electrode structure is mounted on rods 15 sealed in an end. not shown. of the tube envelope.

Referring to FIG. 2 the dimensions shown relate to a known type of triode. corresponding parts having the same reference numerals as in FIG. I. The dimensions are in inches. The length of the coated cathodes and grid or gate electrodes is 3 inches.

FIG. 3 shows a schematic view. in partial section. of a tube having an electrode construction similar to that of FIG. I inserted between a pair of polepieces 20 and ZI of a magnet not otherwise shown. In FIGS. 1 and 2 like parts are identified by the same respective numerals. The support rods 15. appearing dotted in FIG. 3. are spaced apart with the aid of insulators l6 and 17 which also support the cathode filaments. not visible in FIG. 3. The electrode structure is mounted from a ceramic base 22 and is surrounded by a water jacket 23, cooling water flowing between the anode and the wall of the water jacket via inlet and outlet pipes one of which is shown at 24. The water jacket, the anode 11, the grid plates 12 of FIG. 1 as well as the support plates 14, are all made of non-magnetic material so that a virtual air gap exists between the polepieces and 21.

In order to adjust the strength of the magnetic field across the air gap between polepieces 20 and 21 a magnetic shunt 25 is provided. The position of this shunt with respect to the polepieces 20 and 21 is made adjustable, as indicated by the arrow 26, so as to shunt the main air gap between the polepieces to a greater or lesser extent.

When the shunt 25 is at a distance from the polepieces such that it exerts negligible shunting effect on the magnetic flux, the electrode potentials may be ad justed to provide maximum power output, the focusing magnetic field strength then also having a maximum value, I'I for example. As the shunt is moved towards the polepieces, no alteration being made to voltage controls, the power output falls with a decreasing field across the air gap, as indicated in FIG. 6 in which power output is plotted against magnetic focusing field strength. For a typical tube providing a power output of kW with a magnetic field of 1,200 gauss, a reduction of the focusing field to 600 gauss is found to reduce the power output to 3 kW without significant increase in beam interception by the grid control plates.

Referring to FIG. 4, the presently preferred magnet structure comprises polepieces 27 and 28 (corresponding to 20 and 21 in FIG. 3) which provide facing pole faces and which are mounted on a U-shaped permanent magnet assembly 29 by bolts such as 27a. Assembly 29 comprises two U-shaped parts 30 and 31 secured to a base part 32 by means of a through clamping bolt 33. Magnetic shunting plate parts 34 and 35 are slidably mounted in slots (not shown) in base 32 and each has an elongate slot 36 or 37 to clear the through bolt 33.

Each part 34 and 35 extends into a control base 38 mounted on the base 32 by means of plates 39 and 40. A control member comprising a spindle 41 carries a gear wheel (not shown) engaging teeth (not shown) on the edges of those portions of parts 34 and 35 in the box 38. Thus by rotation of spindle 41 the parts 34 and 35 can be moved towards and away from the polepieces 27 and 28 and the magnetic field therebetween to thus vary the magnetic field strength. The tube would sit on the base in the space between the pole faces. The figures quoted above and the graph of FIG. 6 apply to the structure of FIG. 4 using a typical known tube in the circuit of FIG. 5.

The circuit diagram shown in FIG. 5 is a tuned anode, coupled grid oscillator suitable for induction heating work. It is shown with the work coil in series with the anode tank coil to give a high output impedance, but the output may equally well be taken from the secondary coil of a work-head transformer to provide a low impedance output. The primary of the transformer is connected across the high impedance output points.

The anode tank circuit forms the largest unit. Because of the large r.f. currents involved. it is desirable to construct it with as few joints as possible. The tank capacitance C is made up of six 4,500pF capacitors which are bolted down to a base plate and have their center lugs connected to a water-cooled bus bar. The connectors are formed from the bus bar so as to provide good thermal paths to assist in cooling the capacitors. A tank coil L which consists of four 9 inch (228.6mm) diameter turns of 0.5 inch (12.7mm) copper tube, is coupled directly to the water connection on the capacitor bus bar so that the coil also is watercooled. The other end of the coil is taken to the base plate via a work coil L or primary of the work-head transformer. The whole coil and busbar assembly are preferably silver-plated in order to keep circuit losses to a minimum. The tube and its magnet are mounted on insulators and are coupled to the tank circuit by a 9,000pF capacitor C The high voltage is fed to the tube via an inductance L of 2,500,u.H which consists of a 3 feet (91.44 cm) long single layer solenoid of number 16 gauge enamelled copper wire closely wound on a 3 inch (76.2mm) diameter former.

The feedback to the grid is from a coupling coil L which consists of four 11 inch (279.4mm) diameter turns of inch (9525mm) copper tube. This is mounted so that it surrounds the anode coil in the maximum coupling position and can be tilted away from the anode coil to reduce the coupling. This coil is wound in the opposite sense to that of the anode coil and the end remote from the tank capacitors is connected directly to the base plate. The other end is connected to the gate terminal via a 1,000 pF capacitor and an antiparasitic choke formed from ten turns of number 16 wire on an 0.5 inch (12.7mm) diameter former. The

d.c. return the for grid is taken via a lmH choke L to resistor R, which is between 6kQ and 8kQ, to watts.

The anode is fed from a source of d.c. voltage V,, between 6 and 8kV and the cathodes are grounded. The density of the magnetic flux B directed from the cathode to the anode is variable.

When the tube is oscillating, grid current flows for a fraction of the oscillation cycle depending upon the time constant of the circuit. Electrons reach the anode during corresponding periods of the oscillation cycle, being gated off by the negative grid potential at other times. The power output depends upon the peak anode current and the period of anode conduction and is dependent on the grid swing and the constants of the grid circuit. If an attempt is made to control the power output by altering the value of the bias resistor R1, only a small change of output power from its maximum value with fixed anode supply voltage and feedback coupling is obtained before oscillation ceases. If, however, the magnetic flux density B through the valve is varied, the output power may be changed over a very wide range, as is illustrated in the curve of FIG. 6 in which power output in kilowatts is plotted against flux density B in gauss. A d.c. supply voltage of 7.3 kV and a magnetic field of 1,200 gauss reduces grid current to a low value under static conditions (the curve relating grid current to magnetic flux density falling only slowly with increasing flux density). Prior to the invention, with this tube the magnetic flux density was normally set at 1,200 gauss, a power output of about 30 kW being obtained at 400 kilohertz for a conduction angle of 1 15 and a peak grid current of 0.6 amps and mean grid current of 54 mA. Reducing the focusing field to 850 gauss is quite surprisingly found to reduce the output power to 13.6 kW, the conduction angle increasing to and the peak grid current falling to 0.4 amps, the mean grid current being 36 mA. The dc. kilovoltamps supplied fell from 46.15 to 32.

These results were entirely unexpected, and although it is believed that the construction of the particular tube may be a contributing factor, similar results may be achieved with, for example, a tube having circular concentric electrodes, radially extending grid electrodes and a variable radially extending magnetic field. A valve arranged to operate with a magnetic field directed normally to the path of the electron beam or any other direction, provided it is still arranged to minimize grid current, may also exhibit this phenomena. In another variation, instead of or in addition to the permanent magnetization, an electromagnetic arrangement may be used together with electrical control means operable to vary the strength of the magnetic field by controlling the current through an electromagnetic windmg.

It should be pointed out that when reference is made above and in the claims to do. potentials applied to the tube, this does not necessarily mean that the actual electrode potentials remain unaltered as the magnetic field is changed; for, depending upon the actual circuit used, the anode and grid or gate currents may vary with the variation of magnetic field so that the mean potentials of electrodes change even though the external voltage settings are kept constant.

The present invention thus provides a surprisingly simple means of adjusting the output power of a magnetically focused thermionic tube smoothly and over a much wider range than is obtained by complicated and expensive control equipment hitherto used.

What is claimed is:

l. A thermionic electron tube comprising an anode electrode, a cathode electrode spaced from said anode and providing an electron beam therebetween, a control grid electrode disposed alongside said cathode and electron beam, means providng a magnetic field focusing said electron beam, the magnetic field surrounding said electrodes and directed substantially parallel to the electron beam path, and means for adjusting the magnetic field strength to vary the power output from said anode.

2. The device of claim 1 comprising an open loop shaped permanent magnet having facing pole faces on opposite sides, said tube and electrodes being positioned between said opposite sides, a shunt of magnetic material disposed adjacent said magnet between said opposite sides, and a control member connected with the shunt and operable to selectively move said shunt toward and away from said pole faces to shunt the magnet to an adjustable extent to vary the strength of the magnetic field and the power output of the tube.

3. The device of claim 2 wherein said shunt is substantially U-shaped, the limbs of the shunt having free ends at said opposite respective sides.

4. The device of claim 3 including means supporting said magnet, each said limb being slidably mounted in a slot in said supporting means.

5. The device of claim 1 wherein said anode includes walls surrounding said cathode and grid, said cathode including a plurality of spaced longitudinal parallel wires, and said grid includes a plurality of spaced longitudinal parallel plates respectively positioned alternately between said cathode wires, said plates being aligned parallel with said electron beam to minimize interception thereof. 

1. A thermionic electron tube comprising an anode electrode, a cathode electrode spaced from said anode and providing an electron beam therebetween, a control grid electrode disposed alongside said cathode and electron beam, means providng a magnetic field focusing said electron beam, the magnetic field surrounding said electrodes and directed substantially parallel to the electron beam path, and means for adjusting the magnetic field strength to vary the power output from said anode.
 2. The device of claim 1 comprising an open loop shaped permanent magnet having facing pole faces on opposite sides, said tube and electrodes being positioned between said opposite sides, a shunt of magnetic material disposed adjacent said magnet between said opposite sides, and a control member connected with the shunt and operable to selectively move said shunt toward and away from said pole faces to shunt the magnet to an adjustable extent to vary the strength of the magnetic field and the power output of the tube.
 3. The device of claim 2 wherein said shunt is substantially U-shaped, the limbs of the shunt having free ends at said opposite respective sides.
 4. The device of claim 3 including means supporting said magnet, each said limb being slidably mounted in a slot in said supporting means.
 5. The device of claim 1 wherein said anode includes walls surrounding said cathode and grid, said cathode including a plurality of spaced longitudinal parallel wires, and said grid includes a plurality of spaced longitudinal parallel plates respectively positioned alternately between said cathode wires, said plates being aligned parallel with said electron beam to minimize interception thereof. 